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Optik 125 (2014) 5196–5198

Contents lists available at ScienceDirect

Optik

jo ur nal homepage: www.elsev ier .de / i j leo

ealization of free space optics with OFDM undertmospheric turbulence

ushank Chaudharya,∗, Angela Amphawana,b, Kashif Nisara

InterNetworks Research Laboratory, School of Computing, University Utara Malaysia, MalaysiaResearch Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, USA

r t i c l e i n f o

rticle history:eceived 7 October 2013

a b s t r a c t

Free space optics (FSO) technology provides a promising solution for future broadband networks, offeringhigh data transmission compared to RF technology. This work is focused on investigating the performance

ccepted 5 May 2014

eywords:ree space optics (FSO)FDMemiconductor optical amplifier (SOA)og

of an FSO system with OFDM and QAM. A 10 Gbps data stream is transmitted using a 4-level QAM sequencethrough the FSO system under different atmospheric conditions. Results indicate that the integration ofSOA prolongs the maximum achievable distance with acceptable SNR to 185 km under clear weatherconditions whereas under atmospheric fog, the maximum distance is extended to 2.5 km.

© 2014 Elsevier GmbH. All rights reserved.

. Introduction

The growth of internet traffic in conjunction with an increase inhe number and range of new services has placed pressure on radioetworks operating on low-speed infrastructure. Free space opticsFSO) has the combined features of prevalent telecommunicationechnologies, i.e. wireless and fiber optics. FSO communication hasttracted significant attention recently in high data rate wirelessinks and provided the essential combination of qualities requiredo bring traffic to the optical fiber backbone. FSO technology is also aromising solution for the “last mile” problem. FSO networks can besed as information bridges between nodes in local area networksnd wireless local loops [1]. In addition, FSO provides secure trans-ission because of negligible interception using point-to-point

aser signals. High capacity, low power consumption, license-freend low deployment costs are some of the merits of FSO [2,3]. FSOas similar working principles as fiber optic communication, theain difference being the use of the atmosphere instead of opti-

al fiber as the channel. An FSO network can be implemented as aoint-to-point, mesh or point to multipoint architecture [4]. Lowower infrared beams, not harmful to the human eye, can trans-

it data through the air between links comprising distance of

ew meters to several kilometers [5]. FSO provides good solutionsor broadband networks, especially in geographical areas where

∗ Corresponding author.E-mail address: sushankchaudhary@gmail.com (S. Chaudhary).

ttp://dx.doi.org/10.1016/j.ijleo.2014.05.036030-4026/© 2014 Elsevier GmbH. All rights reserved.

optical fiber deployment is not feasible but there are some per-formance limitations. The most dominant limiting factors areatmospheric conditions such as fog, dust, snow or rain that debil-itates the transmission path and may close down the network.Thus, in the design of FSO systems, these atmospheric parame-ters must be considered [6]. The link equation for free space optics[7] is

PRecieved = PTransmittedd2

R

(dT + �R)210−˛R/10 (1)

where dR defines the receiver aperture diameter, dT is the transmit-ter aperture diameter, � is the beam divergence, R is the range and

is the atmospheric attenuation. In FSO links, multipath fading isanother limiting factor.

Significant effort is imperative for reducing multipath fadingdue to atmospheric turbulence. OFDM has immense potential formitigating multipath fading due to atmospheric turbulences in FSOas data is distributed over a large no of orthogonal carriers that aresufficiently spaced at narrow frequencies with overlapping bands.The use of fast Fourier transform (FFT) provides orthogonality to thesubcarriers, preventing the demodulators from seeing frequenciesother than their own. The application of OFDM offers higher datacapacity, secure transmission, high speed and smooth upgrade [8].

The rest of the paper is organized into following sections: Section2 describes the simulation setup for OFDM–SOA system and Sec-tion 3 describes the result and discussion. The paper is concludedin Section 4.

S. Chaudhary et al. / Optik 125 (2014) 5196–5198 5197

Fig. 1. Block diagram of proposed FSO network.

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Fig. 3. Evaluation of (a) SNR vs. range. (b) Total received power vs. range under clearweather conditions.

Fig. 4. Evaluation of (a) SNR vs. range. (b) Total power vs. range under differentatmospheric conditions.

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Fig. 2. Generation of modulation format. (a) ODSB. (b) OSSB.

. System description

The proposed OFDM–FSO system is modeled usingptiSystemTM from Optiwave Corp. 10 Gbps data is generated

hrough 4-level QAM sequence generator using 2 bits per symbol.he QAM data signal is modulated by an OFDM modulator using12 subcarriers, 1024 FFT and 32 cyclic prefix code before beingodulated at 7.5 GHz using a QAM modulator as shown in Fig. 1.

his QAM signal is transmitted over free space by means of aontinuous wave (CW) laser having a wavelength of 193.1 THz andower of 0 dBm.

The FSO network consists of pre- and post-amplification inhich SOA is incorporated for post-amplification. At the base

tation, the OFDM signals are retrieved using a PIN photodetectornd fed to the QM demodulator followed by the OFDM demodu-ator and QAM sequence decoder in order to recover the 10 Gbpsata successfully.

The optical modulation formats are shown in Fig. 2. OSSB isenerated by using a phase shift in the optical modulator.

. Results and discussion

The results are presented in this section. Fig. 3 depicts theeasurement of SNR and received power for ODSB and OSSBodulation formats under clear weather condition by considering

ttenuation of 0.11 dB/Km. It has been observed that after 20 km, anmprovement of 3.2 dB is measured in the case of ODSB. The OSSB

echnique suffers more severely from fading. After 60 km, both theSSB and ODSB perform equally well.

The total received power after 20 km is measured as −51.46 dBmnd −54.64 dBm for ODSB and OSSB respectively. With the

able 1valuation of SNR and total power under different atmospheric conditions.

Range (km) Haze Thin fog Light fog

SNR (dB) Power (dBm) SNR (dB) Power (dBm) SNR (dB)

0.5 50.11 −44.13 49.70 −44.67 49.54

1 48.91 −45.76 48.21 −46.79 47.90

1.5 48.14 −46.89 46.91 −48.73 46.38

2 47.73 −47.94 45.69 −50.71 45.05

2.5 46.76 −48.97 44.71 −52.60 44.19

Fig. 5. Constellation diagram after 3 km FSO transmission link under different atmo-spheric conditions. (a) Haze. (b) Thin fog. (c) Light fog. (d) Moderate fog. (e) Heavyfog.

incorporation of SOA, the FSO link under clear weather conditions

prolongs to 180 km with acceptable SNR and BER. Fig. 4 depictsthe measurement of SNR and total received power under differentatmospheric weather conditions. The typical values of attenua-tion with corresponding visibilities is considered as 4 dB for Haze,

Moderate fog Heavy fog

Power (dBm) SNR (dB) Power (dBm) SNR (dB) Power (dBm)

−44.89 49.38 −45.10 48.75 −45.99−47.24 47.58 −47.72 46.02 −50.15−49.56 45.85 −50.43 44.10 −54.37−51.89 44.52 −53.06 40.11 −59.88−54.04 43.86 −55.54 25.33 −74.75

5198 S. Chaudhary et al. / Optik 12

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ig. 6. RF spectrum after 3 km FSO transmission link under different atmosphericonditions. (a) Haze. (b) Thin fog. (c) Light fog. (d) Moderate fog. (e) Heavy fog.

dB/km for thin fog, 13 dB/km for light fog, 16 dB/km for thick fognd 22 dB/km for heavy fog [9,10].

Table 1 illustrates the value for SNR and total power for differentanges of FSO transmission link under all atmospheric conditions.

The constellation diagrams of the proposed OFDM–FSO– sys-em under different atmospheric conditions are shown in Fig. 5. Its reported that the signal strength decreases as the atmosphericttenuation increases.

It is observed that the RF power decreases as the atmosphericonditions vary from low attenuation to higher attenuation, i.e.rom haze to heavy fog. The power of the RF spectrum is computeds −30 dBm for haze, −40 dBm for thin fog, −60 dBm for light fog,

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5 (2014) 5196–5198

−85 dBm for moderate fog and −100 dBm for heavy fog. It is alsoreported from the constellation diagram Fig. 5(e) and RF spectrumFig. 6(e) that the FSO link cannot prolong to 3 km under heavy fogcondition.

4. Conclusion

In this work, a novel system for 10 Gbps OFDM based on FSO isdesigned with the integration of the semiconductor optical ampli-fier (SOA). From our results, it is concluded that with the use of pre-and post-amplification technique, the FSO system will prolong to185 km under clear weather conditions with acceptable SNR andreceived power. When the atmospheric attenuation is increasedand reaches to heavy fog conditions, then the achievable distanceis extended to 2.5 km with acceptable SNR and BER.

References

[1] B. Binder, P. Yu, J. Shapiro, J. Bounds, An atmospheric optical ring network, IEEETrans. Commun. 38 (January (1)) (1990) 74–81.

[2] H. Willebrand, B. Ghuman, Free Space Optics: Enabling Optical Connectivity inToday’s Networks, Sams, Indianapolis, IN, 2002.

[3] L. Andrews, R. Phillips, C. Hopen, Laser Beam Scintillation with Applications,SPIE, Bellingham, WA, 2001.

[4] V.W.S. Chan, Free-space optical communications, J. Lightw. Technol. 24 (12)(2006) 4750–4762.

[5] H.A. Willebrand, B.S. Ghuman, Fiber optics without fiber, IEEE Spectrum 38 (8)(2001) 40–45.

[6] Z. Jia, Q. Zhu, F. Ao, Atmospheric attenuation analysis in the FSO link, in: Inter-national Technology on Communication Technology (ICCT), 2006, pp. 1–4.

[7] S. Bloom, E. Korevaar, J. Schuster, H. Willebrand, Understanding the per-formance of free-space optics, J. Opt. Networking June (2) (6) (2003)178–200.

[8] Hamed Al Raweshidy, Shozo Komaki, Radio Over Fiber Technologies for MobileCommunications Networks, Artech House, London, 2002.

[9] E.J. McCartney, Optics of the Atmosphere, Wiley, New York, 1969.10] I. Kim, B. McArthur, E. Korevaar, Comparison of laser beam propagation at

785 nm and 1550 nm in fog and haze for optical wireless communications, Proc.SPIE 4214 (2000) 26–37 (Optical Wireless Communications III, edited by Eric J.Korevaar).